91 



88. If now, instead of consider- Conditions of 



ing a single cell, we turn our attention to a large plant like a f Highly! 1E 

 tree which has, as we have already seen., a very complicated Organised 

 structure, we begin to realise the necessitv for its variously plant ' 



o ** / 



differentiated parts. If the body of such a plant were com- 

 posed of an aggregate of thin-walled cells, it is obvious that 

 turgidity alone would not suffice to enable it to stand erect and 

 to resist the enormous strains to which it is subjected. Many 

 cells therefore are deprived of their protoplasm and are converted 

 into thick-walled elements such as fibres, which serve to strength- 

 en and support the plant body. Further, although food 

 materials in solution could diffuse through such a large plant 

 body if it were composed entirely of thin- walled living cells, by 

 osmosis from cell to cell, the currents thus produced would be too 

 slow to satisfy the plant's requirements and hence we find 

 that some cells are converted into conduction pipes, such 

 as the vessels and tracheids, which are able to conduct water 

 and salts rapidly from the roots to the leaves, and the sieve- 

 tubes which conduct the organic food from the leaves to the 

 places where it is required. By means of a system of inter- 

 cellular spaces, also, which communicate with the outer air, 

 fresh currents of carbon dioxide and oxygen are continually 

 brought into contact with the walls of the living cells, while 

 the water vapour exhaled by the latter is able to quickly pass off 

 into the outer air. That water is actually absorbed by the 

 roots and is given off by the leaves of a highly-organised 

 plant can be easily seen, if the uninjured roots of a young 

 plant are put in an air-tight vessel of water, the whole 

 being placed in a balance. .The amount of water then taken 

 up by the roots can be directly measured, while the amount 

 given off by the leaves is given by the loss of weight indicated 

 by the balance. 



If the external conditions remain constant, the amount of 

 water absorbed by the roots is equal to the amount given off 

 by the leaves. 



The fact that green plants can grow and develop 

 normally in pure sand, or distilled water, to which certain 

 mineral salts have been added, but which contains no carbon, 

 shows that the carbon required for their organic food is not 

 obtained from the soil. The further fact that starch is actually 

 formed by .the green leaves is also easily shown by the follow- 

 ing experiment. 



Part of a healthy, living leaf on a plant which has been kept 

 in the dark for some hours is covered with a piece of tinfoil, 



